Surface-normal optoelectronic devices, i.e. those for which the flow of light is perpendicular to the surface of the device, are typically less expensive to produce and package than waveguide devices in which the flow of light is parallel to the device surface. Testing of surface-normal devices can be performed easily at the wafer level, increasing yield and reducing cost. In packaging, alignment tolerance is favorable due to the relatively large optical windows of surface-normal devices in comparison with the small thickness of active layers in planar waveguide devices, or compared to the core diameter of optical fibers. For lasers, the emitted optical mode may be circular, which adapts well to the core of an optical fiber, and makes vertical cavity surface emitting lasers more attractive. Finally, surface-normal devices may be easily arranged in arrays for multi-fiber connections, or for displays such as liquid crystal displays.
The need for surface-normal optical modulators, e.g., those in which optical reflectivity may be modulated by an electrical signal, arose with proposals of fiber-to-the-home systems based on recirculation of light from the home to the central office. Typically, these proposals are for wavelength multiplexed passive systems which utilize an optical modulator at the subscriber location to replace the active LED or laser devices proposed in prior art systems. See, for example, L. Altwegg, A Azizi, P. Vogel, Y. Wang, and P. Wiler, “LOCNET—a fiber-in-the-loop system with no light-source at the subscriber end”, J. of Lightwave Tech., vol. 12, no. 3, pp. 535–540, 1994; also see: N. J. Frigo, P. D. Magill, T. E. Darcie, P. P. Iannone, M. M. Downs, B. N. Desai, U. Koren, T. L. Koch, C Dragone, and H. M. Presby, “RITE-Net: A passive optical network architecture based on the remote interrogation of terminal equipment,” Proc. of the Optical Fiber Conference—post deadline session, (San Jose, Calif., Feb. 20–25, 1994) pp. 43–47. This approach has several advantages, among them lower cost and higher reliability. The passive devices are also less sensitive to temperature variations, and have a robustness suitable for the uncontrolled environment at some customer locations. Additionally, wavelength routing in the network is more reliable since the upstream light is identically the same wavelength as the downstream light. It also allows easier diagnostics of failures in the system. With a customer based light source system, if the central exchange ceases receiving signals from the customer location, the cause can be either be a breakdown of the customer's laser or a fiber break. With a recirculating system, the cause can only be in transmission, i.e. a fiber break.
A significant advantage of a recirculated system is that it can be multiplexed with several wavelengths (wavelength division multiplexing, WDM), thus increasing capacity. WDM offers attractive system flexibility since in the field a passive optical device, such as a wavelength grating router, may direct each particular wavelength to a particular home. For a typical fiber-to-the-home system, a single fiber may be strung to multiple customer locations, and a high bandwidth common channel serves all of those customers. It is also generally believed that the splitter that separates the individual signals from the common channel should be passive to reduce cost and increase reliability of the system. In a bidirectional system, a passive splitter also functions as a signal combiner for signals from the customer to the central exchange (upstream). If the customer location is provided with a laser source, as in a non-recirculating system, that laser must be an expensive single frequency device with precisely controlled output, and controlled environment, to match the wavelength assigned to the downstream signal. However, this function is relatively simple in a WDM recirculating system since the upstream wavelengths are inherently precisely the same as the downstream wavelengths.
Therefore, it is evident that recirculating systems are preferable for versatile WDM systems. In these systems, there is a need for an inexpensive, robust modulator that operates over a range of wavelengths and, for reasons explained above, is a surface-normal device. There is a need in the art for surface-normal modulators with response times shorter than 10 μs, and with high contrast ratio and wide optical bandwidth.
To meet these needs, a low-cost silicon optical modulator has been developed based on micro electro mechanical systems principles (MEMS). The device has been designated MARS, for Moving Anti-Reflection Switch. This device has a movable conductive membrane suspended over a conductive substrate. With an appropriate electrostatic field the membrane is controllably moved toward, or away from, the substrate thus producing a precisely controlled air gap between the membrane and the substrate. With proper positioning of the membrane with respect to the substrate the MARS device can be switched from a reflecting state to an anti-reflecting state. For more details of the basic MARS device see K. W. Goossen, J. A. Walker, and S. C. Arney, “Silicon modulator based on Mechanically-Active Anti-Reflection layer with 1 Mbit/sec capability for fiber-in-the-loop applications, “IEEE Phot. Tech. Lett., vol. 6, pp. 1119–1121, Sep. 1994.
The basic MARS structure is made by forming an approximately 1 mm-thick film of phosphosilicate glass (PSG) on a silicon substrate and an approximately 0.2 mm-thick film of silicon nitride on top of the PSG. The nitride film forms the movable conductive membrane and the silicon substrate forms the conductive substrate. Details of the MARS structure and its fabrication can be found in U.S. Pat. No. 5,500,761. Much of the PSG layer is sacrificial, i.e. after the silicon nitride layer is formed part of the PSG layer that is between the silicon nitride layer and the silicon substrate, is etched away leaving a portion the silicon nitride “floating”, resulting in the air gap that allows movement of the silicon nitride layer with respect to the silicon substrate. Composite polysilicon/silicon nitride/polysilicon films can also be used to advantage for the movable membrane. See U.S. Pat. No. 5,654,819, which is incorporated herein by reference for the details of that MARS structure and for the spatial relationships that define the MARS elements. Advantages of the MARS device are ease of fabrication, which leads to low manufacturing cost, wide spectral bandwidth, and high speed. Disadvantages of the MARS structures include charging of the suspended silicon nitride membrane (especially when used alone, i.e. without polysilicon coatings), and residual stress in the membrane itself, which can lead to undesirable deformation or curling. Membranes with double support beams, such as those used in channelized spectral equalizers, are particularly susceptible to lateral curling. This results in large dead zones in the membrane which functionally produces large unwanted pass bands in the optical signal.